1. Field of the Invention
The present invention relates to an acoustic transducer (i.e., electroacoustic converter) which radiates a sound wave into air or water, and in particular to an acoustic transducer capable of efficiently radiating a sound wave at a low frequency.
2. Description of Related Art
An acoustic transducer which radiates a sound wave into a medium, such as water, is used in the field of oceanographic observation or the like. As the frequency of a sound wave to be used is low, attenuation is small and a propagation characteristic is excellent, such that acoustic radiation can be carried out over a long distance. For this reason, in recent years, an acoustic transducer which radiates a sound wave having a low frequency by excluding many mediums, such as water, in the vicinity of the acoustic radiation surface, has come into practical use.
As the acoustic transducer of the related art which is used in water, various types including a bolted Langevin type transducer, a cylindrical transducer, a flextensional transducer, a bending disc-type transducer, a barrel stave-type transducer, and the like are currently used.
According to “Fundamentals and Applications of Marine Acoustics (edited by Marine Acoustics Society of Japan)”, Seizando, Apr. 28, 2004, p. 58-60 (hereinafter, referred to as Non-Patent Document 1), a bolted Langevin transducer (also referred to as a tonpilz transducer due to its shape) 101 shown in FIGS. 14A and 14B is configured such that an acoustic radiation plate 104 is provided on one end surface of a vibrator module 103 having a plurality of annular piezoelectric vibrators 102. The vibrator module 103 causes acoustic radiation from the acoustic radiation plate 104 using a vibration mode where the vibrator module 103 longitudinally vibrates in half wavelength.
A cylindrical transducer 111 shown in FIGS. 15A and 15B is configured such that an acoustic radiation plate 114 is provided on the outer circumferential surface of a cylindrical vibrator 112. The cylindrical transducer 111 causes acoustic radiation from the acoustic radiation plate 114 using a breathing vibration mode in a radial direction of the cylindrical vibrator 112, that is, using a mode where longitudinal vibration with one wavelength is formed on the circumferential length of the cylinder.
A flextensional transducer 121 shown in FIGS. 16A and 16B is configured so as to expand the amplitude by converting vibration of a vibrator 122 into bending vibration of a bending acoustic radiation plate 124 using an elliptical shell having an elliptical sectional shape and to cause acoustic radiation of the displacement of the vibrator 122 by using flexural vibration of the bending acoustic radiation plate 124, instead of directly radiating a sound wave into water by using resonance of the vibrator.
According to Japanese Unexamined Patent Application, First Publication No. H5-344582 (hereinafter, referred to as Patent Document 1), a bending disc-type transducer 131 shown in FIGS. 17A and 17B is configured such that a disc-type vibrator 132 is bonded to a bending acoustic radiation plate 134, and flexural resonance of the bending acoustic radiation plate 134 is used, thereby causing acoustic radiation of the displacement of the vibrator 132.
The acoustic transducers 121 and 131 respectively employ the bending acoustic radiation plates 124 and 134 which use bending vibration whose resonance frequency is easily obtained with a low frequency compared to longitudinal vibration, such that many mediums can be excluded.
According to U.S. Pat. No. 4,922,470 (hereinafter, referred to as Patent Document 2), a barrel stave-type transducer 141 shown in FIGS. 18A and 18B is configured such that a plurality of bent acoustic radiation plates 144 are provided at the outer circumferential portion, and a gap d1 is provided between adjacent acoustic radiation plates 144.
Meanwhile, an electrodynamic loudspeaker (i.e., acoustic transducer) which is generally used in air transfers vibration of a coil by an electromagnetic force to a cone paper, thereby causing acoustic radiation from the cone paper.
With regard to acoustic radiation into air, acoustic radiation impedance is small, thus it is possible to secure a large excluded medium volume with a lightweight material, such as paper.
However, the acoustic transducers of the related art have the following problems.
In the bolted Langevin transducer 101 shown in FIGS. 14A and 14B and the cylindrical transducer 111 shown in FIGS. 15A and 15B, in order to increase the excluded medium volume, it is necessary to increase the displacement of the acoustic radiation plate by increasing the length or diameter of the vibrator, which causes an increase in the size and weight of the acoustic transducer Thus, at present, these acoustic transducers are used with a frequency greater than or equal to 1 kHz due to the limitations on size and the like.
In the flextensional transducer 121 shown in FIGS. 16A and 16B and the bending disc-type transducer 131 of Patent Document 1 shown in FIGS. 17A and 17B, in order to increase the excluded medium volume, it is necessary to increase the area of the acoustic radiation plate. In this case, the acoustic transducer increases in size and weight.
In particular, when piezoelectric ceramic having a large mass is used for the bending vibration plate, a structure is made such that a large amplitude location has a large mass, and a low resonance frequency is obtained. However, the weight may increase and the degree of sharpness of the resonance frequency may be high, such that this type of transducer is not suitable for acoustic radiation over a wide band.
In the barrel stave-type transducer 141 shown in FIGS. 18A and 18B, a gap is required between adjacent acoustic radiation plates. For this reason, if the entire transducer is molded for watertightness, the vibration of the gap d1 may be disturbed due to water pressure and the efficiency of acoustic radiation may be deteriorated.
With regard to the electrodynamic loudspeaker which is used in air, in order to increase the excluded medium volume, a larger cone paper is used, and as a result, the loudspeaker increases in size. Further, like the piezoelectric loudspeaker, when the piezoelectric vibrator is bonded to the vibration plate to form acoustic radiation, in order to increase the excluded medium volume, it is necessary to increase the diameter of the vibration plate.
When an acoustic transducer is provided in an underwater sailing body or a towing body, it is desirable that the specific gravity of the acoustic transducer is close to or smaller than the specific gravity of the medium (such as water). If the specific gravity of the acoustic transducer is greater than that of the medium, a floating buoyant material for floating the acoustic transducer is required, in the case of the underwater sailing body, and the acoustic transducer drops in the case of the towing body where there is no space for providing a floating buoyant material. In the acoustic transducer of the related art where acoustic radiation with a low frequency is possible, there are many cases where piezoelectric ceramic is used for the vibrator, and the specific gravity of the acoustic transducer is usually greater than or equal to one.
Further, an acoustic transducer which is used in a towing body is accommodated to be cylindrically wound at the time of being accommodated and is substantially used in a linear shape at the time of operation. For this reason, at the time of being accommodated, large bending stress is applied to the acoustic transducer, such that the acoustic transducer is damaged.